Synthesis of new steroidal inhibitors of P-glycoprotein-mediated multidrug resistance and biological evaluation on K562/R7 erythroleukemia cells

The roles of the human ATP-binding cassette transporters P-glycoprotein and ABCG2 in multidrug resistance in cancer and at endogenous sites: future opportunities for structure-based drug design of inhibitors

The ATP-binding cassette (ABC) transporters P-glycoprotein (P-gp) and ABCG2 are multidrug transporters that confer drug resistance to numerous anti-cancer therapeutics in cell culture. These findings initially created great excitement in the medical oncology community, as inhibitors of these transporters held the promise of overcoming clinical multidrug resistance in cancer patients. However, clinical trials of P-gp and ABCG2 inhibitors in combination with cancer chemotherapeutics have not been successful due, in part, to flawed clinical trial designs resulting from an incomplete molecular understanding of the multifactorial basis of multidrug resistance (MDR) in the cancers examined. The field was also stymied by the lack of high-resolution structural information for P-gp and ABCG2 for use in the rational structure-based drug design of inhibitors. Recent advances in structural biology have led to numerous structures of both ABCG2 and P-gp that elucidated more clearly the mechanism of transport and the polyspecific nature of their substrate and inhibitor binding sites. These data should prove useful helpful for developing even more potent and specific inhibitors of both transporters. As such, although possible pharmacokinetic interactions would need to be evaluated, these inhibitors may show greater effectiveness in overcoming ABC-dependent multidrug resistance in combination with chemotherapeutics in carefully selected subsets of cancers. Another perhaps even more compelling use of these inhibitors may be in reversibly inhibiting endogenously expressed P-gp and ABCG2, which serve a protective role at various blood-tissue barriers. Inhibition of these transporters at sanctuary sites such as the brain and gut could lead to increased penetration by chemotherapeutics used to treat brain cancers or other brain disorders and increased oral bioavailability of these agents, respectively.

Androstano-arylpyrimidines: Novel small molecule inhibitors of MDR1 for sensitizing multidrug-resistant breast cancer cells

Apart from the numerous physiological functions of MDR1, it is widely known for its role in granting multidrug resistance to cancer cells. This ATP-driven transmembrane protein exports a wide range of chemotherapeutic agents from cancer cells, thereby deterring drugs to reach effective intracellular concentrations. Thus, inhibition of MDR1 expression or function would be a viable option to enhance the accumulation of cytotoxic agents in cancer cells which in turn could improve significantly the success rate of chemotherapy. Although, several pharmacological inhibitors have been designed and tested in the past, due to their unsuccessful translation to clinical application, there is still ongoing research to find suitable compounds to manipulate MDR1 function and potentially overturn multidrug resistance. In the present study, we demonstrate that novel DHT-derived A-ring-fused arylpyrimidinone derivatives, based on their acetylation status, can inhibit MDR1 efflux activity in MDR1 overexpressing multidrug-resistant breast adenocarcinoma cells. Strikingly, all derivatives carrying an acetoxy group on the sterane d-ring were highly potent in hindering Rhodamine 123 export via MDR1, however deacetylated molecules were not capable to exert a similar effect on multidrug resistant cancer cells. The possible molecular and cellular mechanisms underlying the efflux pump inhibiting function of acetylated derivatives were dissected using the most potent MDR1 inhibitor, compound 10g and its deacetylated counterpart (11g). Importantly, molecule 10g was able to sensitize drug resistant cells to doxorubicin-induced apoptosis, further verifying the highly advantageous nature of efflux pump inhibition upon chemotherapy. Our experiments also revealed that neither mitochondrial damage, nor MDR1 gene regulation could lay behind the MDR1 inhibitory function of compound 10g. Molecular docking studies were carried out to analyze the interactions of 10g and 11g with MDR1, however no significant differences in their binding properties were observed. Nevertheless, our results indicate that the ER stress inducing potential of molecule 10g might be the fundamental mechanism behind its inhibitory action on MDR1. With additional studies, our work can yield a structural platform for a new generation of small molecule MDR1 inhibitors to sensitize drug resistant cancer cells and at the same time it elucidates the exemplary involvement of endoplasmic reticulum stress in the molecular events to defeat multidrug resistance.

Pharmacodynamic mechanisms of anti-inflammatory drugs on the chemosensitization of multidrug-resistant cancers and the pharmacogenetics effectiveness

Drug resistance as a remarkable issue in cancer treatment is associated with inflammation which occurs through complex chemical reactions in the tumor microenvironment. Recent studies have implicated that glucocorticoids and NSAIDs are mainly useful combinations for inflammatory response modulation in chemotherapeutic protocols for cancer treatment. Immunosuppressive actions of glucocorticoids and NSAIDs are mainly mediated by the transrepression or activation regulation of inflammatory genes with different DNA-bound transcription factors including AP-1, NFAT, NF-κB, STAT and also, varying functions of COX enzymes in cancer cells. Interestingly, many investigations have proved the benefits of these anti-inflammatory agents in the quenching of multidrug resistance pathways. Numerous analyses on the ABC transporter promoters showed conserved nucleotide sequences with several DNA response elements that participate in transcriptional regulation. Furthermore, genetic variations in nucleotide sequences of membrane transporters were strongly associated with changes in these transporters' expression or function and a substantial impact on systemic drug exposure and toxicity. It appeared that several polymorphisms in MDR transporter genes especially MDR1 have influenced the regulatory mechanisms and explained differences in glucocorticoid responses.

Medicinal chemistry strategies to discover P-glycoprotein inhibitors: An update

The presence of multidrug resistance (MDR) in malignant tumors is one of the primary causes of treatment failure in cancer chemotherapy. The overexpression of the ATP binding cassette (ABC) transporter, P-glycoprotein (P-gp), which significantly increases the efflux of certain anticancer drugs from tumor cells, produces MDR. Therefore, inhibition of P-gp may represent a viable therapeutic strategy to overcome cancer MDR. Over the past 4 decades, many compounds with P-gp inhibitory efficacy (referred to as first- and second-generation P-gp inhibitors) have been identified or synthesized. However, these compounds were not successful in clinical trials due to a lack of efficacy and/or untoward toxicity. Subsequently, third- and fourth-generation P-gp inhibitors were developed but dedicated clinical trials did not indicate a significant therapeutic effect. In recent years, an extraordinary array of highly potent, selective, and low-toxicity P-gp inhibitors have been reported. Herein, we provide a comprehensive review of the synthetic and natural products that have specific inhibitory activity on P-gp drug efflux as well as promising chemosensitizing efficacy in MDR cancer cells. The present review focuses primarily on the structural features, design strategies, and structure-activity relationships (SAR) of these compounds.

In vitro modulation of multidrug resistance by pregnane steroids and in vivo inhibition of tumour development by 7α-OBz-11α(R)-OTHP-5β-pregnanedione in K562/R7 and H295R cell xenografts

Synthetic progesterone and 5α/β-pregnane-3,20-dione derivatives were evaluated as in vitro and in vivo modulators of multidrug-resistance (MDR) using two P-gp-expressing human cell lines, the non-steroidogenic K562/R7 erythroleukaemia cells and the steroidogenic NCI-H295R adrenocortical carcinoma cells, both resistant to doxorubicin. The maximal effect in both cell lines was observed for 7α-O-benzoyloxy,11α(R)-O-tetrahydropyranyloxy-5β-pregnane-3,20-dione 4. This modulator co-injected with doxorubicin significantly decreased the tumour size and increased the survival time of immunodeficient mice xenografted with NCI-H295R or K562/R7 cells.

Cabazitaxel is more active than first-generation taxanes in ABCB1(+) cell lines due to its reduced affinity for P-glycoprotein

PURPOSE

The primary aim of this study was to determine cabazitaxel's affinity for the ABCB1/P-glycoprotein (P-gp) transporter compared to first-generation taxanes.

METHODS

We determined the kinetics of drug accumulation and retention using [¹⁴C]-labeled taxanes in multidrug-resistant (MDR) cells. In addition, membrane-enriched fractions isolated from doxorubicin-selected MES-SA/Dx5 cells were used to determine sodium orthovanadate-sensitive ATPase stimulation after exposure to taxanes. Custom [³H]-azido-taxane analogues were synthesized for the photoaffinity labeling of P-gp.

RESULTS

The maximum intracellular drug concentration was achieved faster with [¹⁴C]-cabazitaxel (5 min) than [¹⁴C]-docetaxel (15-30 min). MDR cells accumulated twice as much cabazitaxel than docetaxel, and these levels could be restored to parental levels in the presence of the P-gp inhibitor PSC-833 (valspodar). Efflux in drug-free medium confirmed that MDR cells retained twice as much cabazitaxel than docetaxel. There was a strong association (r² = 0.91) between the degree of taxane resistance conferred by P-gp expression and the accumulation differences observed with the two taxanes. One cell model expressing low levels of P-gp was not cross-resistant to cabazitaxel while demonstrating modest resistance to docetaxel. Furthermore, there was a 1.9 × reduction in sodium orthovanadate-sensitive ATPase stimulation resulting from treatment with cabazitaxel compared to docetaxel. We calculated a dissociation constant (Kd) value of 1.7 µM for [³H]-azido-docetaxel and ~ 7.5 µM for [³H]-azido-cabazitaxel resulting in a 4.4 × difference in P-gp labeling, and cold docetaxel was a more effective competitor than cabazitaxel.

CONCLUSION

Our studies confirm that cabazitaxel is more active in ABCB1(+) cell models due to its reduced affinity for P-gp compared to docetaxel.

Asclepiasterol, a novel C21 steroidal glycoside derived from Asclepias curassavica, reverses tumor multidrug resistance by down-regulating P-glycoprotein expression

Multidrug resistance (MDR) mediated by P-glycoprotein (P-gp) is a major cause of cancer therapy failure. In this study, we identified a novel C₂₁ steroidal glycoside, asclepiasterol, capable of reversing P-gp-mediated MDR. Asclepiasterol (2.5 and 5.0μM) enhanced the cytotoxity of P-gp substrate anticancer drugs in MCF-7/ADR and HepG-2/ADM cells. MDR cells were more responsive to paclitaxel in the presence of asclepiasterol, and colony formation of MDR cells was only reduced upon treatment with a combination of asclepiasterol and doxorubicin. Consistent with these findings, asclepiasterol treatment increased the intracellular accumulation of doxorubicin and rhodamine 123 (Rh123) in MDR cells. Asclepiasterol decreased expression of P-gp protein without stimulating or suppressing MDR1 mRNA levels. Asclepiasterol-mediated P-gp suppression caused inhibition of ERK1/2 phosphorylation in two MDR cell types, and EGF, an activator of the MAPK/ERK pathway, reversed the P-gp down-regulation, implicating the MAPK/ERK pathway in asclepiasterol-mediated P-gp down-regulation. These results suggest that asclepiasterol could be developed as a modulator for reversing P-gp-mediated MDR in P-gp-overexpressing cancer variants.

Deoxycholic acid derivatives as inhibitors of P-glycoprotein-mediated multidrug efflux

Deoxycholic acid derivatives were designed as P-glycoprotein (Pgp, ABCB1) inhibitors. Thus the synthesis and the biological activity of methyl deoxycholate derivatives 5-10 and their ether analogs 15-20 have been reported. The potency of these compounds to modulate Pgp-mediated MDR was evaluated through daunorubicin accumulation and potentiation of doxorubicin cytotoxicity in K562/R7 multidrug resistant cells overexpressing Pgp. In parallel, their intrinsic toxicity was appreciated on K562 sensitive cells. Methyl 12α-[(2R or 2S) tetrahydro-2H-pyran-2-yloxy]-3-oxo-5β-cholan-24-oate 9b has shown a good efficiency as a Pgp inhibitor and a low intrinsic toxicity. Therefore, this derivative constitutes a new lead compound which can be used as a starting point to improve the design of non-toxic Pgp modulators.

Biotransformation of 6-dehydroprogesterone with Aspergillus niger and Gibberella fujikuroi

Microbial transformation of 6-dehydroprogesterone (1) with Aspergillus niger yielded three new metabolites, including 6β-chloro-7α,11α-dihydroxypregna-4-ene-3,20-dione (2), 7α-chloro-6β,11α-dihydroxypregna-4-ene-3,20-dione (3), and 6α,7α-epoxy-11α-hydroxypregna-4-ene-3,20-dione (4), and two known metabolites; 6α,7α-epoxypregna-4-ene-3,20-dione (5), and 11α-hydroxypregna-4,6-diene-3,20-dione (6). Compounds 2, and 3 contain chlorohydrin moiety at C-6, and C-7, respectively. The biotransformation of 1 with Gibberella fujikuroi yielded a known compound, 11α,17β-dihydroxyandrosta-4,6-dien-3-one (7).

First synthesis of 22-oxa-chenodeoxycholic acid analogue

In this study, we report the first synthesis of 22-oxa-chenodeoxycholic acid analogue via androstenedione and progesterone, in 11 and 8 steps with overall yields of 6.4% and 12.7%, respectively. We anticipate this will help to facilitate the development of new drugs.